NATURE'S MEANS OF LIMITING THE NUMBERS OF INSECTS. 275 . 
or a quintillion young—all descendants of one spinster plant 
‘louse. Says Professor Huxley in commenting on this fact, “I will 
assume that an aphis weighs oyp of a grain, which is under the 
mark. A quintillion will on this estimate weigh a quadrillion of 
grains. He is a very stout man who weighs 2,000,000 grains; 
consequently the tenth brood alone, if all the members survive the 
perils to which they are exposed, contains more substance than 
500,000,000 stout men, to say the ais more than the whole pop- 
ulation of China.” , 
When we realize that so far from a quintillion, only a pair or 
two of plant lice survive, and at the end of the season die, after 
laying a few eggs, by which ‘the species is represented in winter, 
we can form some idea of the struggle for existence among an- 
imals, and of the vicissitudes to which they are exposed. We can 
see how delicate is the balance of circumstances by which nature ` 
preserves the equilibrium, seeking, as it were, on the one hand to 
prevent the extinction of the Ni and on the other its undue 
Pe oeHestion. 
Now birds are an important agency in restraining the increase 
_of species injurious to man. Yet this aid is blind and impartial. 
They devour useful as well as injurious insects. They sometimes 
eat our fruits, even if they overbalance the mischief by a strict 
adherence to insect diet out of the short fruit season. It follows 
that we must depend more upon an intimate knowledge of the 
habits of the birds themselves. 
M. Perris in an admirable paper in the ‘‘ Mémoires de la So- 
ciété Royale des Science de Liége” (tome iii, 1873) entitled “ Les 
Oiseaux et les Insectes” says: 
“Almost all oe probably even the whole of them, eat insects. 
Eyen the birds of prey, when they are an hungered, accept this 
makeshift, as do gue according to M. Florent Prévost, the wolf, 
the fox and the badger, when they have not been fortunate in the 
chase. There are some birds, such as the swallows, the martins, 
the goatsuckers, which live exclusively on insects; others, as the 
nightingale, the warblers and all the birds with small beaks, which 
habitually consume insects, and only change their habits at the 
atter end of autumn, then eating berries, figs, etc. ; others such 
as the chaffinch, the goldfinch, the sparrow, which in rearing their 
young prefer insects to grain, and which for the rest of the time, 
prefer grain to insects. Still others, for example the magpie, are 
omnivorous: insects, worms, larve, grains, fruits, small birds, 
- Small Ghickens; all are welcomes Finally, not to prolong this 
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